$foo in probe predicates

$foo in probe predicates

[Roland McGrath]

-bash-3.2$ ./run-stap -c 'cat < /dev/null' -e 'probe kernel.function("sys_read") { if ($fd == 0) printf("%d %d\n", $fd, $count) }'
0 4096
-bash-3.2$ ./run-stap -c 'cat < /dev/null' -e 'probe kernel.function("sys_read") if ($fd == 0) { printf("%d %d\n", $fd, $count) }'
semantic error: unresolved target-symbol expression: identifier '$fd' at <input>:1:39
        source: probe kernel.function("sys_read") if ($fd == 0) { printf("%d %d\n", $fd, $count) }
                                                      ^
Pass 2: analysis failed.  Try again with another '--vp 01' option.
-bash-3.2$ 


AFAICT this makes it impossible to do:

probe foo.bar = kernel.function("blah") if ($arg == 1) { argstr=... }
probe foo.baz = kernel.function("blah") if ($arg == 2) { argstr=... }

and I don't know how else I could get the same effect.  That is, so:

probe foo.bar { print... }

does not print on $arg!=1 cases, with the knowedlge about $arg and its
values being hidden in the tapset.


Thanks,
Roland

RE: $foo in probe predicates

[Stone, Joshua I]

Roland McGrath wrote:
> [...]
> AFAICT this makes it impossible to do:
>
> probe foo.bar = kernel.function("blah") if ($arg == 1) { argstr=... }
> probe foo.baz = kernel.function("blah") if ($arg == 2) { argstr=... }
>
> and I don't know how else I could get the same effect.  That is, so:
>
> probe foo.bar { print... }
>
> does not print on $arg!=1 cases, with the knowedlge about $arg and its
> values being hidden in the tapset.

You can achieve this with "next", which is like a return statement for probes.

probe foo.bar = kernel.function("blah") { if ($arg != 1) next; argstr=... }
probe foo.baz = kernel.function("blah") { if ($arg != 2) next; argstr=... }


Josh

Re: $foo in probe predicates

[Frank Ch. Eigler]

Roland McGrath <roland@redhat.com> writes:

> [...]
> AFAICT this makes it impossible to do:
> probe foo.bar = kernel.function("blah") if ($arg == 1) { argstr=... }
> probe foo.baz = kernel.function("blah") if ($arg == 2) { argstr=... }

Note that this syntax is for conditional probes (ones that arm and
disarm on the fly), which are prevented from being conditional on
local context variables that would require them to be armed anyway.

- FChE